Low capacity switch



June 3, 1952 055 2,599,314

LOW CAPACITY SWITCH Filed July 2, 1947 2 SHEETS-SHEET 1 INVENTOR M4?- Wd iln/M ATTORNEYS June 3, 1952 L. J. BOBB 2,599,314

LOW CAPACITY SWITCH Filed July 2, 1947 2 SHEETS-SHEET2 7? ATTORNEYSPatented June 3, 1952 LOWCAPAOITYfSWITCI I I Lloyd .J'. -.Bobb,;.,Glcnside, .Pa;,,assign 1m 4 Internatio'nalv Electronics. (lo mpany-,-A Philadelphiag Pa a corporation of; Pennsylvania- 8 'Claims. 1;.

Thisinventi'on:relates t electricalswitchestand more particularly 1 hasi to do with electrical switches for use wliereit is essential toreducethe capacity between the" circuits being-switched to a;

Itis frequently necessar-yin electronic work to provide" for'alternativelycoupling a circuit element such as a magnetic recordinghead or a speaker microphone to the input and output of a highfgai'nv'acuumtube amplifier; The difference ineriergyievels between the inputand output of such amplifiers may be very high. Forexample, inmagneticrecording' amplifiers, the ratio of voltagesl at the output andinput may be as high as onemillion toone. It will be understood,therefore; that the transfer of even a very slight amount of energy fromtheoutput circuit to the input circuit will result in oscillations. Evena very. small, amount of capacity between switch contactsjmay result inthe transfer of sufficient energy to produce such an-effect.

A principal object of. theinvention is an electricalswitchinwhich thedirect capacity between theacti ve and inactive contacts is at aminimum.

Afurther object of the invention is to provide for simultaneousswitching of a plurality of circuits ;while retaining different portionsof the circuits imelectrostatic isolation from one another.

The invention also contemplates. an electrical switch adapted tomassproduction and assembly from a minimum number of simple components.

Howthei foregoingand'other objects are attainedgwill be' mo're' clearlyvunderstood from the description,hereinbelow" and the drawings, in.

which:

Figure-,1 is a transverseesectional view of a switch constructed'inaccordance with the invention, taken alongpthe, line 1-1 in Figure 2;

Fig-uref2is: an axialsection .of the switch of Figures; 1 g. and:

Figure'Bfis aschematic diagramof one application'of the switch of theinvention.

Essentially, the switch of the invention takes theformoi a plurality offixed contacts arranged in a-circle and a link shielded from oneanother, and comprising a pair of electrically interconnected movingcontacts {adapted to engage two of'ftlie fixed. contacts simultaneously,together wi'th a shield surrounding the link, to eliminatecapacitycoupling between the moving contacts andltheactivefixed contactson the one hand and the inactive contacts on the other.

'In;the; embodiment of. the invention illustrated in thegdrawings; theswitchcomprises two similar 2:1 1 witch sections. Each; section... as;ma most clearly be seen in Figure 1, comprises four-.-ex;= tern'allythreaded'fixedxcontactpmembers 23113281), 290, and 20d;These:contacts,:lie;in ,a-'; Dlane=.:i

- opposed pairs;xlwithptheraxes oi the -pairs intersecting. The locusof:.the.-'inner ends 'pf the contacts is -a circle: Fixed contactmembers-% 2%, 26c and-=2 di'are adiustablyif u potted yuiflw ternallythreaded; insulating bnshingszzfl, which bushings =in turn aresupported'xby an external cylindrical shield ;m'emb'erqi2 5::Each:.-contact; :26 extendsthrough. external shield-.2 5cto a point justshort of the innen surface-inf:-ithe'ashiel'ds- Disposedwithin thecirclevisla coiledaspring-ZI mounted within. an."insulatingsblushing 'iibetween a pair of: ball contact 11183110613223 zsup ported by i aninternal. cylindrical-shield: member 26 which is mounted for: rotation:withinr mem;- ber '25. i As .will .be seen iromsFigure; 1, spring :2!urges ballmembeias' 23' outwardly againstthaiinner surface of externalis1eevel25: Advantageously, spring 2i. maybepermanently..ztconnectedtwith contactmembers 23,. as-bysoldering;

Whe'ndnternal; shield 26 isurotated until-spring 2 i and bushing 22 arealigned with a-pair ofzfixed contact members Zihrfor example ivithfixed-con:- tacts 2 Eta and; 2&0, ball members 23 are urged .iintocontact with fixed :contactmembers."2iiaiand '28c, thusestablishing an;electrical circuit through fixed Contact 2 Ga, ball: 23,; spring; 2:!the second ball 23 and fixed contactl2ilci Y In orderv to "ensurecentering-of. the 'switcl'u: each fixed contact 26 :is provided atitsjnner: vendwith a depression :21 of spherical contour adaptedlt'oreceive .balls 23.. Thus a's :internal sleevet me'm ber 26 is i rotated.ball. members-:23: rideagainst the inner. surface of: external .sleevcza and then snap into depressions121.:.

It will beseen that'in thevpositi'on represented by :Figure' 1,.theelectrical .circuit.betweehicnne tact 28a and.contact.20c;istc'ompletelyshielded fromboththe inactive contacts z'fihand zod by theinternalshield member; 2 6i andaexternal'shield member 25'... The twoshieldzmembers. 25 and 26 maybe constructed of anyv conductive material,but preferably of material havinghigh elect zical conductivity, such as,for example,,brass.=

As will also. be seen from Figural, I.have;-pro.- vided individualshields.28 forthe external; leads toeach of the fixed contacts. Theseexternal shields may be bonded. to-the fiexible shield 2,9 of the wire30 by whichgthe switch is connected to other circuit elements.

In Figure 2, I have illustrated two switch,.,sejc- .tions of..thetypedisclosed in Figure 1 gangedto provide for simultaneousswitching of a plurality of circuits. In this figure, internal shieldmember 26 will be seen to be mounted on a shaft 31, which shaft carriesa knob 38. In addition, external shield member 25 is provided with athreaded bushing 39, adapted to penetrate a supporting panel 40 and tobe secured in place by a nut 4|. It will also appear from Figure 2 thatnot only are the contacts of a single switch section shielded from oneanother as was described above in connection with Figure 1, but that theseveral contacts in one section are shielded from the contacts in theother section by the portions of the shield members interposed betweenthe two sections. For example, portion 26a of internal shield 26separates the moving contacts of the two switch sections, and portion25a of external shield '25 shields the fixed contacts from one another.In addition, individual external shield members 28 serve toelectrostatically isolate the external leads from one another.

All of the shield members are in contact with one another, and, when theswitch is installed, are connected to ground, for example, through panel40. Thus, a grounded conducting element is interposed between eachcontact and every other contact, eliminating direct capacity between thevarious circuits connected to the switch. Of course, there is capacitybetween each contact and ground, but since this capacity is not commonto any two circuits, no coupling between the circuits can take place.

The problem of capacitive coupling between the input and outputterminals of a amplifier is particularly acute in magnetic recordingsystems, and accordingly, I have illustrated in Figure 3 the applicationof the invention to a magnetic recording system, although it is to beunderstood that similar problems arise in other types of circuit andthat the invention is applicable to any situation in which it is desiredto eliminate capactive coupling between the several circuits to beswitched. Included in Figure 3 are an amplifier 32, a speaker-microphone33, a magnetic recording-reproducing head 34, and a switch of the typeillustrated in Figures 1 and 2, comprising a first section 35 and asecond section 36. As will be noted from the drawing, one terminal ofeach of the input and output circuits and one terminal of thespeaker-microphone and magnetic recording head is grounded. In addition,I have indicated that the metal parts of the switch sections are alsogrounded.

In this circuit, the high side of the amplifier input is connected toterminals and d of switch section 35, while the high side of theamplifier output is connected to contacts b and c of switch section 36.The speaker-microphone is connected to contact a of section 35 and alsoto contact 11 of section 36. Finally, recordingreproducing head 34 isconnected to contact b of section 35 and to contact a of section 36. Itwill be seen, therefore, that when the switch is adjusted to theposition in which the coiled spring in each switch section is vertical,as is illustrated in the figure, the speaker-microphone 33 will beconnected to the amplifier input through contacts a and c of section 35,and that recordingreproducing head 34 will be connected to the amplifieroutput through contacts a and c of section 36. It will also be seen thatsince the amplifier output is connected through switch section 36 tocontact a, the amplifier output voltage will appear between ground andcontact I) of section 35. If, therefore, appreciable capacity existedbetween contact I) of section 35 and either contact a, c, d, or thecoiled spring of section 35, energy would be transferred from theamplifier output to the amplifier input. But because a large portion ofthe grounded internal shield member of the switch is interposed betweencontact I) and all the other contacts, no such capacity can exist and noenergy transfer can occur.

It will be understood that while I have illustrated in Figure 2 a switchcomprising two sections, a larger number of sections may be incorporatedin a single switch, all of which can be controlled by the same knob.

Thus, according to the invention, any pair of contacts may beinterconnected in a circuit which is electrostatically isolated from thecontacts which are temporarily out of the circuit.

In addition, the invention provides for a toggling or self-centeringaction by virtue of the cooperation of the ball contacts with thedepressions in the fixed contacts.

Furthermore, the switch of the invention is admirably suited to massproduction techniques, since the number of parts is small and all areadapted to simple fabrication and assembly.

Iclaim:

1. An electrical switch comprising at least three fixed contactsexternally tangent to a circle and disposed at equal angular distancestherearound, a connecting link within the circle and in the planethereof comprising two electrically interconnected contacts capable ofinternal tangency to the said circle and adapted to selectively engagetwo of the fixed contacts upon rotation of the link about the center ofthe circle, said rotatable contacts being angularly spaced from oneanother by an angle equal to the angle separating two of said fixedcontacts, a grounded conductive shield surrounding said link and movabletherewith, said shield being constructed and arranged to shield the linkfrom any fixed contact not in engagement with the link, a secondgrounded conductive shield of cylindrical section whose inside diameteris less than the diameter of said circle, said second shield beingpierced by apertures aligned with said fixed contact and adapted to bepenetrated by said interconnected contacts, and dielectric means adaptedto guide said interconnected contacts from a position of engagement withsaid second shield to a position of engagement with said fixed contacts.

2. A switch comprising a rotatable cylindrical electrically conductiveshield element, fixed contact elements radially outwardly displaced fromsaid shield element and angularly displaced from one another in the sameradial plane, and electrically interconnected contact means physicallycarried by said shield and being angularly displaced from one another inthe same radial plane, said interconnected contacts being insulated fromthe shield and being outwardly extendible from the shield to selectivelyinterconnect a pair of fixed contacts to thereby make an electricalconnection between said pair of fixed contacts.

3. An electrical switch comprising an external shielding sleevepenetrated by at least three apertures lying in the same radial plane,an insulating bushing extending to the inner surface of the sleeve ineach aperture, a fixed contact extending to the inner end of eachbushing, an internal cylindrical sleeve concentric with the externalsleeve and mounted for rotation therein,

an insulating bushing mounted in a transverse bore through said internalshield in the radial plane of said apertures, a spring Within saidlastmentioned bushing, a contact member at each end of said springwithin said bushing, said contact members being outwardly urged againstthe inner surface of said sleeve by said spring, and a shaft forrotating said internal shield to bring said contact members intoengagement with two of said fixed contacts and thereby establish acircuit through said spring between said two fixed contacts, whichcircuit is shielded from the fixed contact not engaged by said contactmembers.

4. A construction in accordance with claim 3 in which saidfirst-mentioned bushing is internally threaded and said fixed contactmember is externally threaded to provide for axial adjustment of thecontact in the bushing.

5. A construction in accordance with claim 3 in which the spring andcontact members comprise a coiled spring and a pair of metal spheres.

6. A construction in accordance with claim 5 and further including adepression in the inner end of each fixed contact member adapted toprovide a toggling action in cooperation with the said metal spheres.

7. An electrical switch comprising an external shielding sleevepenetrated by at least three apertures lying in the same radial plane, aconductor insulated from the sleeve in each aperture extending radiallyinwardly to a point just short of the inner surface of the sleeve,dielectric means in the aperture, an internal conductive cylinderconcentric with the external sleeve and mounted for rotation therein, aninsulating bushing mounted in a transverse bore through said cylinder inthe radial plane of said apertures, a spring within said bushing, amovable contact member at each end of said bushing, said contact membersbeing outwardly urged against the inner surface of said sleeve by saidspring, and means for rotating said cylinder to bring said contactmembers into alignment with said apertures, said dielectric means beingarranged to support said contact members in their paths of movementacross said apertures between said internal surface and said conductors.

8. An electrical switch comprising an external shielding sleevepenetrated by at least three apertures, a conductor insulated from thesleeve in each aperture extending radially inwardly to a point justshort of the inner surface of the sleeve, a pair of movable contactmembers within said sleeve, means between said contact members urgingthe said members against the inside surface of said sleeve, means forrotating said contact members into positions of alignment with saidapertures, and dielectric means arranged to support said contact membersin their paths of movement between said internal surface and saidconductors.

LLOYD J. 30133.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,201,881 Bryant May 21, 19402,344,780 Kram Mar. 21, 1944 2,360,219 Goddard Oct. 10, 1944 2,425,010Smith Aug. 5, 1947

